Moisture sorption kinetics

Moisture sorption kinetics by neat epoxies were measured using gravimetric analysis using a Mettler balance which was accurate to +0.05 mg. This technique was described in detail elsewhere 137). Another method was used to monitor the sorp-... 

Finally, the critical relative humidities are dependent on the nature of the solid. For example, the spontaneous dissolution process has been observed for many water-soluble substances at relative humidities significantly below that associated with a saturated solution of the substance in water.Van Campen, Amidon, and Zograf have examined the moisture sorption kinetics of deliquescent solids at relative humidities above what they term the critical relative humidity (RHo), where adsorbed water takes on the character of condensed water and serves as a solvent. It is important to recognize that a highly undesirable process such as deliquescence can occur when it may not be expected (e.g., when RHo < RH < RHs). 

L. Van Campen, G. L. Amidon, and G. Zograli, Moisture sorption kinetics for water-soluble substances I Theoretical considerations of heat transport control, J. Pharm. Sci. 72, 1381-1388 (1983). 

Epoxy glasses aged at 140 °C were subjected to 40 °C/98 % relative humidity moisture penetration. Figure 31 shows the results of this transport experiment. We observed both a decrease of initial sorption kinetics as well as a decrease of equilibrium sorption level as a function of aging time. This supports the idea that during sub-Tg annealing, the resin contrasts and densities, resulting in decreased free volume. 

Additives to starch exert varying effects on the kinetics of water sorption. For example, lipids do not significantly affect the content of adsorbed water. The mode of starch defatting can also influence moisture sorption by molecules of the defatting solvent occupying active centers of sorption.389 The addition of either sucrose or lipids to starch has the same effect on both branches of the hysteresis curve.386,398 Some additives, such as dimethyl sulfoxide or ammonium rhodanide, induce selectivity of the adsorption and solvation of starch 411 Sulfur dioxide accelerates water sorption regardless of the temperature.412 Pretreatment of starch with sulfur dioxide usually increases the water sorption.413 Studies on the sorption of components of water-alcohol mixtures are discussed in Section IV. 

Moisture sorption of microcrystalline cellulose has been studied extensively. Fig. 6A includes the sorption and desorption studies for microcrystalline cellulose. The inserts are plots of moisture content versus time, which approximately represent the kinetics of sorption and desorption at each humidity. The equilibrium adsorption isotherm (Type II) has been fit to the BET equation (C = 16.48, = 0.033 g/g solid) and this curve is... 

The effects of temperature and relative humidity on the kinetics of moisture sorption in epoxy materials for microelectronics encapsulation are not generally known. In a previous paper QJ we examined moisture sorption as a function of temperature under conditions of 100 percent relative humidity. Conjugate sorption measurements were combined with mechanical, dielectric and thermal methods of analysis to examine moisture related micro-structural alterations. 

Farnworth [14] reported a numerical model describing the combined heat and water-vapor transport through clothing. The assumptions in the model did not allow for the complexity of the moisture-sorption isotherm and the sorption kinetics of fibers. Wehner et al [30] presented two mechanical models to simulate the interaction between moisture sorption by fibers and moisture flux through the void spaces of a fabric. In the first model, diffusion within the fiber was considered to be so rapid that the fiber moisture content was always in equilibrium with the adjacent air. In the second model, the sorption kinetics of the fiber were assumed to follow Fickian diffusion. In these models, the effect of heat of sorption and the complicated sorption behavior of the fibers were neglected. 

Li and Holcombe [31] developed a two -stage model, which takes into account water-vapor-sorption kinetics of wool fibers and can be used to describe the coupled heat and moisture transfer in wool fabrics. The predictions from the model showed good agreement... 

The assumptions in his model do not allow for the complexity of the moisture sorption isotherm and the sorption kinetics of fibers. Wehner et a.I [67] presented... 

Of particular importance is the timescale over which diffusion occurs under various conditions of relative humidity (RH) and temperature. The RH determines the equilibrium moisture concentration, whereas higher temperatures will accelerate the moisture sorption process. In order to predict the moisture profile in a particular structure, it is assumed that Fickian diffusion kinetics operate. It will be seen later that many matrix resins exhibit non-Fickian effects, and other diffusion models have been examined. However, most resin systems in current use in the aerospace industry appear to exhibit Fickian behaviour over much of their service temperatures and times. Since the rate of moisture diffusion is low, it is usually necessary to use elevated temperatures to accelerate test programmes and studies intended to characterize the phenomenon. Elevated temperatures must be used with care though, because many resins only exhibit Fickian diffusion within certain temperature limits. If these temperatures are exceeded, the steady state equilibrium position may not be achieved and the Fickian predictions can then be inaccurate. This can lead to an overestimate of the moisture absorbed under real service conditions. 

The conventional approach to perform water vapor sorption kinetics is based on the measnrement of the film sample mass until reaching constant value corresponding to a saturation level in a hydrated environment. For each hydrated enviromnent, the mass of sample is measured allowing to obtain the mass gain, ejqtressed in percent, or, in other word, the water concentration, expressed in mass of water soibed per mass of polymer or in mmol of water soibed per mass of polymer, inside sample. The combination of mass gain at each humidity level leads to build the water vapor sorption isotherm. The water vapor sorption and the isotherm shape are depending on the moisture resistance of polymer and its ability to interact with water. 

Burnett, D. J., Garcia, A. R., and Thiehnann, R 2006. Measuring moisture sorption and diffusion kinetics on proton exchange membranes using a gravimetric vapor sorption apparatus. J. Power Sources. 160, 426-430. 

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